Contaminant in a hydraulic fluid is broadly defined as any substance that impairs the proper functioning of a hydraulic system. Hydraulic fluid can be contaminated by air, particles, water, and foreign fluids. Fluid contamination can cause numerous problems including component damage, unacceptable noise, poor component response and severe fluid degradation. This paper focuses on two major contaminants that should be considered when designing a hydraulic reservoir – air and particle contamination. A proper reservoir design can prevent the occurrence of air and solid contaminants within the hydraulic system and reduce their negative effects. A hydraulic reservoir should be designed in such a way as to stabilize and direct the oil flow inside the reservoir, so that the fluid has enough time to release air bubbles and to deposit solid particles. In order to visualize and understand flow patterns inside the reservoir, all the advantages of using simulation techniques within the field of reservoir design will be shown. This paper investigates the trajectories of solid and gaseous particles within a hydraulic reservoir, which are based on simulated transient phenomena using the Ansys Workbench. The results obtained focus on the sedimentation of solid particles and the elimination of gaseous particles within a hydraulic reservoir.